This invention relates generally to an x-ray tube operating in a magnetic field. More particularly, it relates to an x-ray tube having sensors, internal magnets, and/or active compensation means operatively coupled thereto or integrated therein for aligning the cathode-anode axis of the x-ray tube with a magnetic field.
Magnetic resonance imaging (MRI) and x-ray fluoroscopic imaging are important imaging tools. For example, in medical imaging they are routinely used for diagnosing diseases and for image-guided interventional procedures. Each method provides its own advantages: MRI provides excellent soft tissue contrast, three-dimensional visualization, physiological information, and the ability to image in any scan plane, while x-ray imaging offers much higher spatial and temporal resolution in a projection format, useful for visualization and placement of guidewires, catheters, stents, and other medical devices. Combining the two imaging systems therefore offers significant benefits over using each system alone. Currently, several approaches are used for combining the systems. In one, an x-ray fluoroscope is located in a room adjacent to the MRI system. In another, the x-ray and MRI systems are in the same room, but the patient must be moved out of the magnetic field to be imaged by the x-ray system. Moving the patient is undesirable, because it is time consuming, possibly dangerous, and can render the images inconsistent. Therefore, one wants to minimize the distance between the two systems, and perhaps overlap them. This will place critical components of the x-ray system within a high magnetic field.
The ideal system is one in which x-ray imaging and magnetic resonance imaging can be performed in the same location, eliminating the need to move the patient. Before a combined MRI and x-ray system can be constructed, however, the individual systems must be modified to ensure that the high magnetic field of the MRI system does not affect the x-ray system, and that the x-ray system does not disturb the operation of the MRI system. For example, conventional x-ray fluoroscopy detectors are image intensifiers, which are exceedingly sensitive to magnetic fields and therefore cannot be used near, let alone inside, an MRI system. However, flat panel x-ray detectors that are relatively immune to magnetic field effects are now available.
A major obstacle to combining MRI and x-ray systems is the x-ray source, which consists of an x-ray tube and its housing. X-rays are generated using an x-ray tube, in which electrons are accelerated from a heated cathode to an anode by a very high potential (e.g., 50 to 150 kV). Interactions between the high energy electrons of the beam and atoms of the anode target material cause deceleration of the electrons and production of x-ray photons.
FIG. 1 is a schematic diagram of a fixed anode x-ray tube 10 of the prior art. The tube 10 is evacuated and contains a tungsten filament cathode 12 and a more massive anode 14, typically a copper block 16 with a metal target 18 plated on or embedded in the copper surface. The target 18 is most often tungsten, but other metals can be used, such as molybdenum, rhodium, silver, iron, or cobalt. Separate circuits are used to heat the filament 12 and to accelerate the electrons to the target 18. The accelerating potential determines the spectrum of wavelengths (or photon energies) of the emitted x-rays. A high voltage is connected between the cathode 12 and anode 18 to provide the accelerating potential. Typically, the anode and cathode voltages are maintained at plus and minus half of the total accelerating voltage, respectively. X-rays generated at the target 18 exit the tube 10 through an x-ray transparent window 20 and are directed toward the object being imaged.
When an x-ray tube is operated within or near an MRI system, it experiences the static magnetic field Bo, as illustrated schematically in FIG. 2. The magnetic field at the location of the x-ray tube exerts a force on moving electrons and may deflect or defocus the electron beam. The force on an electron is proportional to the cross-product of the velocity of the electron and the magnetic field; that is, only the velocity component that is perpendicular to the magnetic field is perturbed. This will alter the direction of the electron motion, thereby making the direction of the deflecting force time-dependent. In the example of FIG. 2, the electrons are emitted from the cathode with some initial velocity and are accelerated toward the anode by the electric field E. The macroscopic result of the time-dependent force is to produce a deflection away from what would be observed without Bo present, with a deflection in the direction of Bo, and an additional deflection of the beam vxe2x8axa5drift in a direction perpendicular to both Bo and the electric field E. Because the ideal electron velocity is in the direction of the target, as is due to the acceleration caused by the electric field, unless the magnetic field is parallel to the electric field, it will deflect the electrons away from the center of the target, possibly causing them to miss the target entirely. Thus the effect of the static magnetic field of the MRI system on the x-ray tube can be highly undesirable and may damage the tube if it is operated under non-ideal conditions, or it may lower the x-ray intensity to a level that is unacceptable. In the combined system, it is not desirablexe2x80x94indeed it may be impossiblexe2x80x94to turn off the static magnetic field before acquiring x-ray images, and so the effect of the magnetic field on the x-ray tube must be addressed.
A number of combined magnetic resonance imaging and x-ray imaging systems are disclosed in the prior art. U.S. Pat. No. 5,713,357, issued to Meulenbrugge et al., discloses a combined system that minimizes or eliminates the distance an object being imaged must be displaced between individual systems. In one embodiment, the object is displaced a small distance along a track between adjacent MRI and x-ray imaging systems with non-coincident fields of view. In another embodiment, the object is not moved and the fields of view of the two systems are coincident, but the x-ray imaging system is moved out of the MRI field of view during MR image acquisition. During x-ray imaging, the x-ray source is either out of range of the static magnetic field, passively shielded from the magnetic field, or positioned so that the electron beam is parallel to the magnetic field. In this alignment, the electron beam should not be deflected by the magnetic field. This technique, however, limits the system in that the x-ray tube must remain fixed at a certain orientation and/or distance with respect to the static magnet. Moreover, Meulenbrugge et al.""s invention does not teach or suggest how to control/maintain the alignment of the electric and magnetic fields in the x-ray tube.
U.S. Pat. No. 6,151,384, issued to Reed et al, describes an x-ray tube that uses a curved magnetic field to steer electrons toward the anode. This tube cannot be used for imaging since it has multiple, relatively large anode targets that produce x-rays. Further, since Reed et al""s magnetic field is curved, their method will not work well when the source is placed in an environment with a strong external field, such as in the neighborhood of an MRI system.
U.S. Pat. No. 5,818,901, issued to Shulz, discloses a combined system with simultaneous MR and x-ray imaging and coincident fields of view. A solid state x-ray detector containing amorphous hydrated silicon, which is not affected by the magnetic field, is used in place of an image intensifier. The x-ray source is positioned far enough from the MR apparatus that the influence of the magnetic field on the x-ray source is slight. Additionally, the influence is reduced further by surrounding the source with a cladding material that shields the source from the magnetic field. The goal of the cladding or shielding is to reduce the magnetic field at the location of the x-ray source to a level where it can be tolerated.
U.S. Pat. No. 6,031,888, issued to Ivan et al., discloses an x-ray fluoroscopy assist feature for a diagnostic imaging device such as MRI or computerized tomography (CT). X-rays are generated using a rotating anode x-ray tube. There is no mention of the effects of the magnetic field on the x-ray source or of any methods to eliminate such effects.
A medical imaging apparatus containing both x-ray radiographic means and MRI means is disclosed in U.S. Pat. No. 6,101,239, issued to Kawasaki et al. The x-ray and MRM systems have coincident fields of view, and the timing of the image acquisition is controlled so that the x-ray pulses occur only when the gradient magnetic fields and RF magnetic fields of the MRI system arc off. There is no mention of minimizing or eliminating the effect of the static magnetic field on the x-ray source.
These prior art references offer two solutions to the problem of electron beam deflection in the x-ray tube by the static magnetic field of the MRI system: shielding the tube or aligning the electron beam with the magnetic field. Sufficient cladding to completely eliminate the effect of the magnetic field on the electron beam may not be feasible. Aligning the tube with the magnetic field also has potential problems including that the type and/or the placement of the x-ray tube may be limited and that it may be difficult to rotate the x-ray tube to different orientations around the patient. X-ray tube inserts typically have components that distort the magnetic field and pose additional difficulties. More importantly, since such alignment has a very small tolerance, it is critically difficult to attain.
Use of correcting magnetic fields in imaging systems is known. For example, in the context of x-ray image intensifiers, U.S. Pat. No. 5,757,118, issued to Kubo, uses a correcting electromagnetic coil placed inside a cylindrical magnetic shield surrounding an x-ray image intensifier tube apparatus in combination with a ferromagnetic thin plate placed in front of a convex input window of the tube to produce an internal magnetic field which allows the removal of the rotational distortion and S-shaped distortion of an output image resulting from an external magnetic field that reaches a region inside the input window. Specifically, a direct current is applied to the electromagnetic coil so that a magnetic field is produced in the direction opposite to, and thus cancels part of, the external magnetic field. Kubo does not teach or suggest how to maintain/control the alignment of electric and magnetic fields. There remains, therefore, a need to control the alignment between electric and magnetic fields.
The present invention addresses this need by providing an x-ray source that can be operated in the presence of a high magnetic field without being inadvertently affected thereby. The x-ray source comprises an x-ray tube having a cathode and an anode and a magnetic means that is produces a magnetic field having magnetic field lines passing from the cathode to the anode to compensate or correct an otherwise undesired magnetic field. Exemplary magnetic means includes an electromagnet or permanent magnets. The electromagnet may be electromagnetic windings or coils, which may be mechanically coupled to the x-ray tube. The permanent magnets may be integrated inside or positioned outside of the x-ray tube.
In an embodiment, permanent magnets are respectively placed behind the anode and the cathode to create a strong magnetic field between the anode and the cathode, hereinafter referred to as the internal magnetic field. In another embodiment, a strong magnetic field is similarly produced by an electromagnetic winding such as a solenoid coil inside or around the x-ray tube. Since the strength of this internal magnetic field is sufficiently strong enough to outweigh the effects of the external field, the x-ray tube so modified is less sensitive to other magnetic fields that are not parallel to the anode-cathode axis.
If the x-ray tube is operating in the presence of an external magnetic field, especially one that is comparable to or stronger than the internal magnetic field generated therein and the internal magnetic field is not aligned with the external magnetic field, the x-ray tube will experience a torque that attempts to align the x-ray tube with the external magnetic field. One embodiment of the present invention solves this problem with a novel mounting arrangement to allow the x-ray tube to align itself with the external magnetic field. Ideally, the mounting of the x-ray tube within the x-ray source housing allows the x-ray tube to pivot about the focal spot, thereby maintaining the alignment of the x-ray focal spot within the x-ray system.
In some cases, it may not be possible to let the x-ray tube rotate into full alignment. The x-ray tube in these cases, according to an aspect of the invention, should be mounted in such a way that the torque it experiences, or at least the direction of the torque, can be sensed. This sensed mechanical force can then be used as an input to determine the amount of current to be applied to the electromagnetic coils arranged to oppose the transverse field. The x-ray tube experiences no torque when the transverse field is zero. When a torque is sensed, the current supplied to the electromagnetic coils is altered in a way to oppose the torque. This feedback process, which can operate essentially in real time, ideally continues until the sensed torque is effectively zero.